1. Field of the Invention
The present invention relates to a constant pressure mechanism for a probe of contact measuring instrument.
2. Description of Related Art
As widely known, a probe (measurement portion) is pressed onto a target surface by virtue of force of coil spring during measurement with a measuring instrument such as dial gauge. However, if the probe is merely pressed by the coil spring, biasing force of the coil spring changes in accordance with flexed degree of the coil spring, which causes change in contact force according to movement of the probe.
More specifically, when the biasing force of the coil spring changes in accordance with movement of the probe, contact pressure of the probe applied to the target surface also changes in accordance therewith, so that the target surface itself can be deformed or the support condition of the workpiece changes by a counterforce corresponding thereto, thus being unable to obtain correct measurement result with reproducibility.
In view of the above problem, the Applicant of the present patent application has proposed a constant pressure mechanism for pressing the probe at an approximately constant force irrespective of movement amount thereof in Japanese Patent Application No. Hei 10-82788 (Japanese Patent Application Laid-Open Publication No., Hei 11-257904) and Japanese Patent Application No. Hei 10-315469.
However, according to the above constant pressure mechanism, though the measurement pressure of the probe can be maintained approximately constant irrespective of movement amount of the probe, the entire dimension of the mechanism can be significantly large when the movement distance of the probe is large and weight of the mechanism can be increased. Therefore, the above constant pressure mechanism is not suitably used for a probe with large measurement distance
An object of the present invention is to provide a constant pressure mechanism of a probe with small outer dimension and light weight and capable of pressing the probe at an approximately constant pressure even when the movement distance of the probe is large.
For attaining an object of the present invention, the present invention is a constant pressure mechanism of a probe for unidirectionally urging the probe in a predetermined direction by virtue of elastic force of a spring. The constant pressure mechanism of a probe having: a first pulley rotatably supported around a central axis by an instrument stationary portion; a rotation force urging mechanism for urging the first pulley in a predetermined rotation direction; and a first wire having a first end fixed to the first pulley and a second end jointed to the probe.
In the present invention, the rotation force urging mechanism may preferably include: a second pulley coaxial with the first pulley and capable of combined rotation therewith; and a second wire having an end fixed to the second pulley and the spring, the spring having one end being fixed to the instrument stationary portion with a free end jointed to the other end of the second wire.
The first pulley and the second pulley may be respectively either a circular pulley or a spiral pulley. The spiral pulley on one side can change effective diameter relative to the wire, thus appropriately adjusting rotation force for the same displacement. For instance, increase rate of the pull strength differs in a condition where the spring is stretched and a condition where the spring is not stretched even for the same displacement. However, since the spiral pulley is used so that the radius relative to the wire length becomes small when the spring is stretched, always constant pull strength can be generated.
The spring may be a coil spring, or alternatively, may be a plate spring.
The rotation force urging mechanism may be composed of a spiral spring having an end being fixed to the instrument stationary portion and the other end being jointed to the first pulley.
In the present invention, a counter mechanism may preferably be connected to an inner end of the probe. The counter mechanism may be a counter pinion meshed with the probe, thus counting by a rotary encoder etc.